BENEFIT PLANS: Defined Benefit Plan, Net Periodic Benefit Cost, Other Postretirement Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Defined Benefit Plan, Net Periodic Benefit Cost
Net periodic benefit cost	$ 10,976	$ 15,781	[1]	$ 11,194
Postretirement Benefits
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	853	1,051		973
Interest cost	2,989	2,643		2,783
Expected return on plan assets	(2,220)	(2,467)		(2,307)
Immediate Recognition of loss from temporary deviation	0	4,216	[2],[3]	0
Amortization of prior service cost	48	47		47
Net periodic benefit cost	$ 1,670	$ 5,490		$ 1,496

[1] The 2018 pension and postretirement non-service costs includes $4.2 million of expense for a temporary deviation from the cost-sharing provisions of the substantive postretirement plan as described in Note 12 - "Benefit Plans."

[2] In 2018, a loss associated with a temporary deviation from the cost-sharing provisions of the substantive plan was recognized in "Other expense, net" on the consolidated statements of income of the companies.

[3] In 2018, a loss associated with a temporary deviation from the cost-sharing provisions of the substantive plan was recognized in "Other expense, net" on the consolidated statements of income of the companies.